1. Field of the Invention
This invention relates to a display control apparatus and method. More particularly, the invention relates to a display control apparatus and method for a display equipped with, e.g., ferroelectric liquid crystal serving as an operating medium for display update, wherein the state of a display updated as by application of an electric field is capable of being retained.
2. Description of the Related Art
Recent advances in increasing computer processing speed have made it possible to apply computers in a variety of fields in which such application was not feasible in the past. Computers can now be used as the nucleus in multimedia systems in which such media as CDs (compact disks), TV (television) and video tape, etc.
This has been accompanied by the need for specifications for accommodating multimedia in flat-panel displays typified by liquid-crystal displays. For example, in order to display a TV picture or a picture from video tape, the number of display colors desired is at least 260,000.
A ferroelectric liquid-crystal display (hereinafter referred to as an "FLCD") using ferroelectric liquid crystal (hereinafter referred to as "FLC") differs from other types of liquid-crystal displays in that it possesses a "memory" capability. This means that the liquid crystal has a characteristic which allows it to retain the state of a display changed by application of an electric field. By virtue of the memory capability, a display device using FLC suffers no decline in contrast regardless of the number of scanning lines, thus making it possible to realize a large-screen, high-definition display. This feature has been exploited to apply FLCDs to displays in DTP (desktop publishing) systems. With an FLCD panel, however, only two states, namely light and dark, can be expressed by a single pixel. In a case where an image, such as a TV image, containing many gray levels and colors is displayed, it is necessary to execute binary digital processing typified by error diffusion processing and to combine a plurality of pixels to increase the number of grays and the number of colors.
The applicant has previously proposed techniques for displaying images binarized by the error diffusion method. For example, see the specifications of CFM501US (application Ser. No. 08/246,720, filed on May 20, 1994); CFM502US (application Ser. No. 08/243,929, filed on May 17, 1994); CFM503US (application Ser. No. 08/248,511, filed on May 24, 1994); CFG118US (application Ser. No. 08/246,724, filed on May 20, 1994); CFO9174US (application Ser. No. 08/061,026, filed on May 15, 1993); and CFO9177US (application Ser. No. 08/062,214, filed on May 18, 1993).
Error diffusion processing involves an operation of the kind shown in FIG. 17. Specifically, the difference in density between input multivalued pixel data and binary data obtained by binarizing the input data is diffused to the input data of neighboring pixels to be processed subsequently, with weighting being applied to these pixels. Density is preserved. Though the number of colors that can be expressed on the FLCD by error diffusion processing is increased, a decline in image quality and resolution with respect to the original image is unavoidable. Accordingly, in order to improve image quality and resolution when a TV picture is displayed using an FLCD, it is required that first and second fields of an NTSC image signal sent at 2:1 interlacing be combined to produce one frame, after which non-interlaced error diffusion processing is executed.
However, even if error diffusion processing is carried out in frame units, image quality deteriorates when 2:1 interlaced scanning according to the NTSC is used as the scanning method in presenting a display on an FLCD. The reason for this is that since error diffusion processing diffuses error to a neighboring line (the bottom line in the example of FIG. 17), an image in which error is not diffused correctly is displayed for a certain period of time when one line is skipped by interlacing.
The situation is as illustrated in FIG. 18, in which F1 and F2 represent items of binarized frame data obtained by error diffusion processing, with F1 being a first frame of binary data and F2 being a second frame of binary data. When these are subjected to 2:1 interlaced scanning, the content of the display changes in the manner shown at (a), (b), (c), (d) in FIG. 18. In the state shown at (c), the second field of the first frame and the first field of the second frame mix together and a lower line to which error should be diffused does not exist regardless of the field. The result is poor expression of half-tones and an extremely unattractive image. Thus, when binary data that has been subjected to error diffusion processing in frame units is displayed on an FLCD, a problem in terms of image quality arises with 2:1 interlaced scanning. For this reason, it has been construed heretofore that better image quality is obtained when a display is presented using non-interlaced scanning.
Generally, when non-interlaced scanning is performed at a frequency of less than 50 Hz using a CRT, flicker is produced. With an FLCD, on the other hand, there is comparatively little flicker, even when non-interlaced scanning is performed at less than 30 Hz, owing to the aforementioned memory capability of FLC. However, when non-interlaced scanning is carried out, crosstalk that is peculiar to a matrix display occurs depending upon the pattern displayed and, as a result, some fluctuation in luminance occurs at 30 Hz. This can lead to even more flicker than is encountered with a CRT.